In teh hte teh hte a hydrometallurgical process, the solvent employed in the extraction of metals is made up by mixing the extractant with the diluent. Since extractants are very expensive and of a great variety, the selection of an exact extractant for use must depend on the specific nature of the preparation process. Among the many extractants, the acidic di(2-ethylhexyl)phosphoric acid(D2EHPA) and 2-ethylhexylphosphoric acid (PC-88A) are extensively used because of their ability to extract a great variety of metal ions, such as in the recovery of vanadium, zinc and zirconium in chemical industries, as well as in the recovery of radioactive uranium, molybdenum and isotopes in nuclear industries. In some circumstances, for instance, in the extraction of uranium in, a small quantity of trioctyl phosphine oxide (TOPO) is added to D2EHPA to enhance the extraction. There are also many kinds of diluents, such as: toluene, cyclohexane and kerosene, among which kerosene, being the less expensive, has been the most frequently used. The extraction solvent prepared in an appropriate concentration has a good stability, which even after use for a long period of time will still retain its extraction property. Therefore, for production operation in a plant such extraction solvent is a non-consumable chemical. However, when the plant is closed down or when special circumstances arise where it is necessary to dispose of the solvent (herein below known as spent solvent), inevitable problems arise regarding how to deal with the large amount of the organic solvent and whether it is possible to recover the valuable extractants. Especially, if the spent solvent contains radioactive elements so that is must be listed as nuclear wastes, then it becomes an even greater burden for the technology and economic storage management under the regulation of the Atomic Energy Law.
In the following, the extraction of natural uranium will be used as an example to explain the management of spent solvent and the possible methods for the treatment of the spent solvent. There are a large number of plants in the world for uranium extraction from wet process phosphoric acid. These plants were built during the period when uranium prices were relatively high. But these plants, after operating for several years, were no longer economically efficient because of the continuous decline in uranium price. Therefore, they had to be shut down or closed altogether. A residue material that was left behind in large amounts during the decommissioning of the plant was extraction solvent. Because of technical difficulties in the treatment, the extraction solvent along with the spent solvent is currently stored. However, storage is not a good solution because of reasons such as high cost, fire and leakage risk. It has been disclosed in the literature that attempts have included three approaches to treatment of solvents and one is by burning the solvents. However, burning must take place under strictly controlled conditions so that the spent solvent is burned at high temperature and destroyed into gas and ashes without secondary pollution. In reality, however, there will still be a problem with this method, that is, any phosphorous compounds formed during the burning operation will corrode the equipment material. A second approach relies on property transfer. The spent solvent, after filtration and purification, can be sold to similar factories. However by this approach, although there might not be any technical problems, the chance of success generally is very slim because of the suitability of the preparation to potential buyers. Now there is the third approach to the recovery process. The extractants are separated from the kerosene and are kept in storage for ready use while the remaining diluent kerosene after purification can be reused in the process. Comparing the economic benefits after the three methods, the recovery approach would be the best for the treatment of spent solvents. Nevertheless, because the relevant technology still remains under development, at present most of uranium extraction plants that have been closed down deal with their spent solvents temporarily by storage.
As described above, since the spent solvents contain D2EHPA and TOPO, methods currently under consideration on how to recover the said extractant still do not go beyond the solution distribution technique and the distillation technique. In the solution distribution method, difference in the solution pressure of the extractants in relation to the different solvents is used to perform the separation function and the utilizable solvents include nonpolar solvents, such as: kerosene, benzene and carbon tetrachloride and polar solvents, such as: water and ethanol. It has been disclosed in the literature that there are methods of separation of D2EHPA and mono(2-ethylhexyl) phosphoric acid(M2EHPA) with the above nonpolar solvents and polar solvents. In the distillation method, the extractants and kerosene are separated by differences in their respective boiling points. However, since the boiling point of kerosene is about 207.degree. C. and that of the extractant about 218.degree. C. (D2EHPA), the two boiling points are very close to each other and the difference is in the range of 15.degree. C. The effect of separation is therefore not very satisfactory. Also because both the boiling points are high and there is a larger consumption of energy, the method has not been popularly employed.